The present invention relates to the field of mobile wireless communication systems and more specifically to methods and apparatus for communication with mobile telephone users (cellular and personal communication systems), mobile wireless data communications, two-way paging and other mobile wireless systems.
In a mobile wireless network, mobile stations (MS) are typically in communications with one base transceiver station (BTS) through up and down radio links. Such ground-based radio links suffer from strong local variations in path loss mainly due to obstructions and line-of-sight attenuation. As MS move from one point to another, their signal path losses go through shadow fading fluctuations that are determined, among other things, by the physical dimension of the obstructions, antenna heights and MS velocity. These variations in path loss, must be taken into account in the design of the up-link and down-link radio link resource allocation.
While communicating with a specific host BTS, MS are frequently within the communications range of other BTS. Statistically, due to the distribution of physical obstructions, the shadow fading path loss fluctuations to such other BTS tend to be only weakly correlated with the path loss fluctuations on the link between the MS to host BTS link. It is therefore possible that a MS, at anyone time and location, has a lower path loss to a different BTS than the one it is communicating with.
In a conventional wireless network using the GSM standard, the base station controller (BSC) manages the radio link resources of the BTS. These resources are determined by the number of transceivers installed at the BTS and the number of radio channels anyone transceiver can handle. For example, in TDMA standards, a radio channel consists of a frequency and a time slot. In CDMA standards, a radio channel is represented by a frequency and one of a number of orthogonal spreading codes.
A BTS has two principal functions, that of controlling the radio links with all MSs within its cell, and relaying traffic between the BSC and the MSs. Relaying traffic includes receiving down-link traffic from the BSC and broadcasting it to MSs using broadcasters and that of receiving up-link traffic from the MSs using radio receivers called collectors and relaying it to the BSC.
In a mobile wireless network with a BSC, the BSC controls the assignment of the radio link resources (including Broadcaster s and Collectors) in the BTSs as well as the operation of the network, and, through the MSC, provides an interface with the Public Switched Telephone Network (PSTN). For generality, the BTS broadcasting and collecting functions can be considered as separate entities. In most existing networks, however, broadcasters (B) and collectors (C) are co-located.
In one example, three base transceiver stations (BTS) include three broadcasters and three collectors where broadcasters and collectors are typically but not necessarily co-located. The broadcasters and collectors have down-links and up-links to the BSC. These links are typically cabled links such as T1/E1 lines. The connection of these links between the broadcasters or collectors with the BSC may be arranged in various configurations such as a star-like pattern, a daisy-chain pattern or in any combination of these or other patterns.
When a connection is setup between a MS and the mobile network, a BSC selects the BTS that has the best radio access to the MS. This setup process includes a series of signal transmissions back and forth between the BSC, the BTSs, and the MSs using up-link and down-link radio control channels. The setup process results in the assignment of dedicated radio traffic and control channels for the up-links and down-links for communications between the MSs and the BTSs. Once these connections are set-up, user traffic, also called payload, can be transmitted between the MSs and the BSC. While the connection lasts, the BTS/BSC controls the operation of the radio traffic channels, including power control, frequency hopping, and timing advance. Also, the BTS/BSC continues to use the radio broadcast channels for operation, maintenance and signaling with all other MSs in its cell.
Users (MSs) communicate with collectors via control up-links and traffic up-links and with broadcasters via control down-links and traffic down-links. A particular broadcaster and collector is called the host broadcaster and the host collector for a particular MS. Together, they perform the function of the host BTS for the particular MS.
As MSs move within a cell and as the average path loss between an MS and its serving broadcaster and collector degrades, existing networks reassign the MS to another BTS (with a broadcaster and collector) that has a lower path loss. This process is called handover or handoff. Prior systems distinguish between hard and soft handover. During hard handover, both the control and traffic radio links between the MS and BTS are terminated and new radio links are set-up between the MS and the new BTS using the radio resources assigned to the new BTS. In case of a handoff failure, the MS and BTS reestablish the control and traffic radio link as it existed before the handoff was attempted. This hard handover is used in GSM networks. In CDMA networks, hard and soft handoff is practiced. In soft handoff, the new radio links are setup before the old links are terminated (make before break operation). CDMA allows simultaneous communications of a MS with a number of BTS during soft handoff.
One technique for maintaining low transmit power during the operation of a mobile radio link is dynamic power control. It may be applied on both the up-link and down-link directions or only in one direction, and it may be performed in an open-loop or closed-loop mode. In open-loop power control mode, the transmit power is determined by system level parameters. In closed-loop power control mode, the power is dynamically set in response to radio link measurements such as distance measurements between the MS and the BTS (as determined by time of arrival measurements), receive signal strength measurements, or error rate measurements.
Another known method to improve network performance is the use of macrodiversity signal combining (also called aggregation). This method uses multiple spaced-apart transmitter/broadcasters and collector/receivers in the BTSs to simultaneously communicate with a MS. The soft handoff practiced in CDMA is such an example. On the down-link, the signal is transmitted from multiple spaced-apart broadcasters using down-link traffic channels. These multiple signals are received by the MS (for example using a rake receiver in CDMA), and combined, to provide a processed signal with a higher level of confidence. On the up-link, multiple spaced-apart receivers/collectors receive the signal transmitted by the MS on up-link traffic channels. These multiple receive signals are then transported to a central location and processed to provide a processed signal with a higher confidence level then any of the individual signals would provide. One disadvantage of macrodiversity combining, when used on the up-link, is the added backhaul associated with transporting the receive signals from multiple collectors to one central location.
While many different wireless networks have been proposed, there is a need for improved wireless networks that achieve the objectives of improved performance and higher density of MSs.